ProRail said pipes had been found attached to railways in more than 20 locations on Tuesday, mainly in rural areas in the middle and the east of the country.
"We discovered materials that had been deliberately placed there. We are therefore now classifying this as sabotage of the railway infrastructure," ProRail spokesperson Joke van der Cruysen said.
King Willem-Alexander was set to open a new session of parliament on Tuesday afternoon with a speech setting out the government's budget plans for 2027.
All trains in the vicinity of the track blockages were halted, while there were also serious disruptions elsewhere, including to international services to Germany.
"It is not yet known who is responsible or what the motive is", ProRail said.
The Dutch intelligence agency AIVD said it was investigating alongside police and prosecutors.
The budget plans have sparked widespread protests in recent months, including from labour unions angry about plans to cut social benefits and farmers arguing that they are being unfairly blamed for nitrogen pollution.
Farmers had been due to protest in The Hague on Tuesday.
The pipes not only disrupted rail travel but also road traffic as several level crossings had to remain closed, ProRail said.
